1. Field of the Invention
The present invention relates to a noise cancellation device that cancels noise from an engine and the like.
2. Description of the Related Art
When automobiles are running, various kinds of noise such as road noise and wind noise invade the automobile cabin. Various measures to attenuate this noise have been taken in order to satisfy people's desire for luxury automobiles and to reduce driver fatigue. For example, a noise cancellation device has been known in which canceling sound supplied from a speaker installed underneath a seat or the like attenuates engine noise generated in the cabin when the engine is running.
FIGS. 5 and 6 are schematic illustrations of structures of known noise cancellation devices that attenuate engine noise invading an automobile cabin.
In the noise cancellation device shown in FIG. 5, a sine wave signal and a cosine wave signal which are synchronized with the engine rotation are generated using a sine wave table and a cosine wave table, respectively. The noise cancellation device adjusts the gains of the sine wave signal and the cosine wave signal and then produces sound corresponding to the combined signal of the sine wave signal and the cosine wave signal from a speaker. The noise cancellation device adjusts the gains to minimize an error signal “e” at a particular listening position.
In the noise cancellation device shown in FIG. 6, the sine wave signal synchronized with the engine rotation is generated by using a phase-locked loop (PLL) circuit. The noise cancellation device filters the sine wave signal with a finite impulse response (FIR) filter and then produces sound corresponding to the filtered signal from a speaker. The noise cancellation device controls the filter coefficients of the FIR filter to minimize the error signal “e” at a particular listening position.
In the known noise cancellation device shown in FIG. 5, for example, when a signal synchronized with the engine rotation is provided as an input, a sine wave signal and a cosine wave signal are generated on the basis of the rising, the zero crossing point, and the like of the input signal by reading data stored in the sine wave table and the cosine wave table. The data from the sine wave table and the cosine wave table are read in response to the engine rotation. Because the engine rotation speed is variable, the amount of calculation increases, which complicates the processing. Moreover, the device will be expensive because a high-performance processor or the like is needed for performing the complicated processing.
In the known noise cancellation device shown in FIG. 6, in order to achieve good sound-deadening characteristics, the number of taps in the FIR filter must be increased. Thus, the size of the device becomes larger and the device becomes expensive. Also, because the filter coefficients of the FIR filter must be controlled in real time, the amount of calculation increases, which complicates the processing.